Evolution with visual extinction A_V of the fraction of molecular DNM gas (grey) or additional CO_sat H₂ (green) in the total H₂ column-density. The DNM is assumed to be 100% molecular. The shaded areas and error bars respectively give the standard deviation in the sample and the standard error of the mean. The black line gives the Gaussian profile of Xu et al. (2016) and the red line shows the best fit by a half-Gaussian to the fraction.